Large-scale dissociation of molecular gas in galaxies by newly formed stars

Molecular hydrogen is thought to form atomic hydrogen on the surface of dust grains in the interstellar medium, and to be dissociated primarily by ultraviolet radiation from hot stars. Observational evidence that molecular hydrogen is indeed dissociated in shells around young OB stars in the Galaxy has been presented recently. The authors suggest that the same dissociation process occurs on the kpc scale in active star-forming regions of some galaxies, and that this dissociation may strongly affect the observed morphology of atomic hydrogen in spiral arms. (author)